A combined sewer carries sewage from homes and businesses and stormwater runoff from streets in the same pipes. Combined sewer overflows (CSOs) can occur during very wet weather when sewer runoff exceeds Willamette River CSO tunnel system capacity. During a CSO event, combined sewage overflows to the river through the outfall pipes that can be seen along the route. All of the projects on the Stormwater cycling tour keep stormwater runoff out of combined sewers to help control CSOs.

The City of Portland completed a 20-year CSO control program in November 2012. Before CSO control, there were an average of 50 CSO events each year. Today, combined sewers overflow no more than four times each winter and once overy three summers.